Polyesters are excellent in mechanical properties and chemical properties, and are used in broad fields including films and sheets for packaging, magnetic tapes, optics and the like. A polyester film is obtained by melt-extruding and then biaxially stretching a polyester. More specifically, a sheet-like object melt-extruded from an extruder is taken over in close contact with the surface of a rotating cooling drum (casting), and subsequently, the sheet-like object is introduced into a stretching roll arranged at a post-stage of the cooling drum, longitudinally stretched and further laterally stretched by a tenter, and then heat-set. Here, in order to enhance uniformity of the thickness of the film, and increase the casting speed, when cooling the sheet-like object melt-extruded from an extrusion nozzle on the surface of the rotary cooling drum, the sheet-like object and the drum surface need to closely contact each other with sufficiently high adhesion. Therefore, as a method for enhancing adhesion between the sheet-like object and the surface of the rotary drum, there has been widely used a method of providing a wire-shaped electrode between the extrusion nozzle and the rotary cooling drum, applying high voltage thereto, generating static electricity on the surface of the unsolidified sheet-like object, electrostatically depositing the sheet-like object on the surface of the cooling drum, and rapidly cooling the sheet-like object, that is, the so-called electrostatic adhesion casting method.
In the electrostatic adhesion casting method, it is known that increasing the charge amount on the surface of the sheet-like object is effective for improving electrostatic adhesion to the cooling drum of the sheet-like object, and reforming a polyester as a raw material to lower its melt resistivity is effective for increasing the charge amount. Moreover, as the method for lowering the melt resistivity, in a manufacturing stage of the raw material polyester, an alkali metal or alkaline earth metal compound is added during esterification or transesterification, for example (for example, refer to Patent Document 1).
As another method of lowering the melt resistivity, a masterbatch having a very low melt resistivity is separately produced by adding a large amount of an alkali metal or alkaline earth metal compound, and is mixed with a polyester as a raw material of the film (for example, refer to Patent Document 2). However, in the production of a masterbatch, there are problems that a contaminant is easily generated, coloring is easily caused, and thermal stability is lowered, by adding a large amount of an alkali metal or alkaline earth metal compound. Also, a sufficiently low value of the melt resistivity for a masterbatch cannot be obtained, and it is necessary to mix a large amount of masterbatch with the polyester of the film raw material.
This is disadvantageous also in terms of cost.